Mineral bleaching process



United States Patent 3,301,695 MINERAL BLEACHING PROCESS Venancio Mercade, Metuchen, N.J., assignor to Minerals & Chemicals Philipp Corporation, Woodbridge, N.J., a corporation of Maryland No Drawing. Filed May 19, 1965, Ser. No. 457,208 18 Claims. (Cl. 106-309) This invention relates to the brightening of minerals, especially clay. More specifically, the invention relates to an improvement in the process for brightening iron-stained minerals by treating the mineral with a permanganate (an oxidizing agent) and thereafter treating the mineral with a hydrosulfurous bleaching reagent (a reducing agent).

The value of clay as a pigment or filler material is related directly to the whiteness or brightness of the clay.

Sedimentary clays, such as the kaolin clays mined in the southern portion of the United States, are usually discolored. It is conventional practice to beneficiate these clays in order to improve their brightness, thereby increasing the value of the clays as filler or pigment materials. This is done by bleaching an aqueous pulp of the clay with a hydrosulfurous compound, such a zinc hydrosulfite. Before being subjected to the reducing bleach treatment, some clays are beneficiated by froth flotation in the presence of reagents which are selective to colored impurities in the clay. The colored impurities which are floated from the clays are largely titaniferous. The subsequent reducing bleach treatment effects removal of ferruginous colored impurities. 1

Some clays, however, are not brightened to commercially acceptable levels by bleaching with a hydrosulfurous compound, even when carried on in conjunction with froth flotation. An example is gray kaolin clay, a unique form of kaolin which has a distinct gray tinge, in contrast to the more familiar kaolin which has an orange tinge. Gray kaolin, unlike domestic white kaolin, is available in abundant supply. However, because of the failure of gray kaolins to respond satisfactorily to conventional beneficiation processes, these clays have very limited commercial value. It has recently been discovered that gray clay can be brightened to unexpectedly high levels by treating an aqueous acidic pulp of the discolored clay with potassium permanganate before bleaching the clay with the hydrosulfuro-us reducing bleach. This process is disclosed and claimed in a copending application of James B. Duke, Serial No. 330,634, filed December 16, 1963.

An object of this invention is the provision of a method for enhancing the effectiveness of a permanganate treatment of iron-stained minerals that are subsequently bleached with a hydrosulfurous bleaching reagent.

A more specific object is the provision of an improved process for brightening discolored clay.

Another object is the provision of a method for treating a pulp of discolored gray kaolin clay before the pulp is treated with potassium permanganate and reducing bleach so as to improve the response of the clay to brightening by the cooperative effect of the permanganate treatment and reducing bleach treatment.

- Briefly stated, in accordance with this invention, an ironstained mineral is bleached and brightened by forming an aqueous pulp of the mineral, incorporating into the pulp a water-soluble titanium compound, preferably a watersoluble titanium compound having a positive valence less 3,301,695 Patented Jan. 31, 1967 than +4, e.g., titanium trichloride (TiCl and thereafter treating the pulp with the permanganate and then hydrosulfurous bleaching compound. The pH of the mineral pulp is below 7 when it is treated with the titanium compound and with permanganate.

The use of the soluble titanium compound catalyzes, or otherwise increases the effectiveness of the permanganate reagent. As a result, minerals treated with the titanium compound prior to being subjected to the action of the oxidation-reduction bleaching action of permanganate and then hydrosulfurous compound are markedly whiter or brighter than the miner-a1 would be if subjected to the oxidation-reduction treatment without preliminary treatment with the titanium compound. To illustrate, a representative sample of flotation beneficiated gray Georgia kaolin could be brightened to a GE. brightness value of 87.9% by treatment with potassium permanganate and then zinc hydrosulfite. When the gray clay was pretreated, in accordance with this invention, with a small quantity of titanium trichloride before treatment with permanganate and then zinc hydrosulfite, the clay was brightened to a GE. brightness of 89.0% using the same quantities of permanganate and zinc hydrosulfite in the process. The resulting improvement in brightness is of great commercial significance since a clay having a brightness of 89.0% is of considerably greater value than a clay havinga brightness of 87.9%.

An essential feature of this invention resides in the use of a titanium compound in which the titanium compound is soluble.

Another essential feature is that the titanium compound is incorporated with the discolored mineral before addition of permanganate. The desired results are not realized with the titanium compound per se since the amount of brightening achieved in this manner is very modest as compared to the brightening achieved when the process of the invention is followed. Further, the desired results are not realized when the titanium compound is incorporated after the pulp is processed with permanganate but before treatment with hydrosulfurous compound. In fact, in the latter case, the use of the titanium compound was detrimental to brightness. These results have led to the conclusion that the use of the titanium compound eifectuates the action of the permanganate compound.

The titanium compounds that are preferably employed in carrying out this invention include water-soluble titanium compounds having a valence of +3 (titanous compounds), water-soluble titanium compounds having a valence of +2, and solutions of peroxy-type titanium compounds. As examples of titanium compounds that are suitable may be mentioned titanium dichloride, titanium trichloride (titanous chloride), especially titanium chloride hexahydrate, corresponding bromides, iodides, fluorides, titanium (III) sulfate, titanium oxysulfate (TiOSO and titanium (III) phosphate. I prefer to use titanium trichloride because of its relatively low cost and commercial availability. Titanium compounds having a valence of +4, e.g., titanium tetrachloride, titanium (IV) sulfate, etc., can be used. However, it is recommended to employ relatively cool pu-lps, for example, pulps that are at ambient temperature, when the permanganate is used with a tetravale nt titanium compound. The reason .manganate has varied with different clay crudes.

is that tetravalent titanium compounds tend to lose their effectiveness when the permanganate treatment is at elevated temperature. Since optimum brightening is obtained when the pulp is treated with permanganate at elevated temperature, it is therefore preferable to use reduced titanium compounds, e.g., titanium trichloride, which are effective with hot pulps.

The quantity of titanium compound employed is within the range of about 0.05 to about 2 pounds per ton of mineral. Especially good results have been realized using 0.2 to 1 pound of titanium compound per ton of mineral.

In putting the invention into practice, the titanium compound can be diluted with water and the solution incorporated with agitation into an aqueous pulp of the mineral being processed. As mentioned, the titanium compound is added before the permanganate is added. The titanium compound can be agitated with the mineral pulp at temperatures ranging from about 50 F. to 200 F. The duration of the agitation does not appear to be important provided the time is suflicient to insure thorough mixing of the titanium compound in the aqueous mineral pulp. Sixty seconds is a recommended time for agitating the mineral pulp with the titanium compound although agitation periods ranging from 1 minute to 3 hours, for example, may be satisfactory. The solids content of the pulp must be sufliciently low to assure that the pulp is fluid. Pulps of 20% to 70% mineral solids are typical.

The preferred permanganate that is used is potassium permanganate, although other water-soluble permanganates, such as ammonium, calcium and sodium permanganate can be used. The permanganate salt is usually employed in amount within the range of /2 to pounds per ton of mineral. The optimum quantity will vary with the mineral 'being treated. For example,

with gray kaolin clays, the optimum quantity of per- The permanganate can be added to the mineral pulp in the form of an aqueous solution. The pulp can be agitated with the permanganate at temperatures of 50 F. to 200 F., preferably at elevated temperature (e.g., 150 F. to 200 F.), and for times ranging from 10 minutes to 70 hours or more. The aqueous phase of the pulp becomes distinctly brown during the agitation and the presence of the characteristic brown color indicates that.

the permanganate is functioning.

The hydrosulfurous bleaching reagent can be any of the usual hydrosulfurous bleaching reagents, e.g., zinc hydrosulfite or sodium hydrosulfite. It can !be hydrosulfurous acid formed in situ by adding sulfur doxide gas to the slip.

Sulfoxylates, which are compounds formed by reaction of aldehydes with metal salts of hydrosulfurous acid, such as sodium formaldehyde sulfoxylate, can "also be used as the hydrosulfurous bleaching compound.

The bleached mineral slurry can be flocced with acid material such as mineral acid or alum if the slurry is not adequately flocced for filtration. The mineral is then filtered and dried or, if desired, the filtered clay can be redispersed, as with a polyphosphate dispersant, and supplied in slurry form.

The process is applicable to the bleaching of various oxidized minerals, e.g., clays, fullers earth, bauxite, talc, asbestos. The process is especially applicable to the bleaching of gray kaolin, which clay material does not respond in the desired manner to conventional bleaching reagents. In applying the process to gray kaolin, the clay can be raw clay as mined, a suitable size fraction of raw clay, or clay which has been partially brightened by the flotation, e.g., the carrier flotation process of US. 2,990,958 to Ernest W. Greene et al.

The following examples are given for illustrative purposes.

In the examples, brightness index refers to values obtained by TAPPI Standard Method T-646m54, as described on pages 159A and 160A of the October 1954 issue of TAPPI (a monthly publication of the Technical Association of the Pulp and Paper Industry). The method measures the light reflectance of a clay sample and thus gives a quantitative indication of its brightness or whiteness.

All reagent quantities are reported as pounds per ton of dry clay feed unless otherwise indicated.

EXAMPLE I A series of oxidation-reduction bleaching tests was performed on a sample of a fine size fraction of flotation beneficiated gray kaolin clay with the purpose of investigating the effect of the presence of a soluble titanous salt (TiCl on the potassium permanganatezinc hydrosulfite bleaching reagents.

A 6977 gram sample of gray Georgia kaolin containing 6000 grams solids and the balance water was blunged at 30% solids in 13 liters of soft water. The pH of the blunged clay was 4.85. The clay was then dispersed partially by adding 300 milliliters of a 5% aqueous solution of sodium carbonate and agitating the system for 30 minutes. This brought the pH to 7.5. One hundred and eighty milliliters of a 5% aqueous solution of 0 brand sodium silicate (72% water and having a Na OzsiO weight ratio of 1:322) was mixed with 72 milliliters of a 1% aqueous solution of Al (SO .18H O. This dispersant mixture was added to the pulp and the pulp agitated for 20 minutes. The pH of the pulp was 7.9. The dispersed slip was degritted by passing it through a 325-mesh screen and the degritted slip was fractionated in a centrifuge to obtain a fine size overflow fraction containing at least by weight of particles finer than 2 microns (equivalent spherical diameter). The pH of the slip of fine size fraction of clay was 8.0 and it contained 20.5% solids. To 2439 grams of the slip, 411 milliliters of soft water was added and the slip conditioned for froth flotation as follows. Minus 325- mesh calcite (Drikalite) was added in amount of grams (about 30% of the weight of the clay in the slip). The slip was agitated for one mintue. Thirty milliliters of a 5% aqueous solution of ammonium sulfate was added to the slip which was conditioned for /2 minute. An emulsion containing 15 milliliters of a 2.5% aqueous solution of ammonium hydroxide, 127 drops (corresponding to 9, pounds per ton of clay) of a 50-50 (weight basis) mixture of crude tall oil and Calcium Petronate was added and conditioned for 5 minutes. Calcium Petronate is the trade name of a 50% solution of neutral oil-soluble petroleum sulfonate in mineral oil. Eightyfive drops (corresponding to 4.5 pounds per ton of clay) of lube oil (Eureka M) was added and the system conditioned for 12 minutes. The pH of the pulp was 8.6 after being conditioned.

The conditioned pulp of gray clay was subjected to froth flotation to float impurities, principally anatase. The clay pulp was given one rougher flotation and the froth was cleaned three times by reflotation without addition of reagents. 1

Samples of the flotation brightened gray kaolin clay were flocced by adding sulfuric acid to a pH of 2.5. The samples were allowed to stand overnight and the supernatant liquid was removed by siphoning. The solids content of the thickened pulps was about 20% (weight basis). The brightness of the flotation beneficiated clay was 81.8% before being bleached with zinc hydrosulfite.

In accordance with this invention, samples of the thickened pulp, each containing 25 grams of clay, were treated with titanium trichloride as follows. A commercial 20% aqueous solution of titanium trichloride was diluted with water to form a 1% TiCl solution. The 1% solution was added to a sample of the thickened clay pulp in a Pyrex beaker and agitated at ambient temperature of 70 F. to 75 F. with a pyromagnetic stirrer for one minute. A 1.25% aqueous solution of potassium permanganate was then added to the titanium trichloride treated clay pulp and agitated for sixty minutes at a temperature and for a time indicated in Table I. Zinc hydrosulfite bleach liquor was then added (5.0 pounds zinc hydrosulfite per ton of clay, dry clay basis). The pulp was agitated with the zinc hydrosulfite for ten minutes. All preparations were then filtered and Washed with 50 milliliters of soft water. The washed clay was dried at 175 F. for one hour, pulverized and the brightness determined.

For purposes of comparison, the permanganate and zinc hydrosulfite bleach treatments were repeated with portions of the flocculated thickened flotation beneficiated clay. The results are also summarized in Table I.

Table I EFFECT OF TITANIUM TRICHLORIDE ON THE BLEACH- ING OF FLOTATION BENEFICIATED GRAY KAOLIN CLAY WITH POTASSIUM PERMANGANATE AND ZINC HYDROSULFITE 1 Aged 48 hours after agitation with KMnO4.

2 Added after KMI104.

RT=room temperature (7075 F.).

The data in Table I show that, all other variables being constant, the use of titanium trichloride before permanganate treatment improved the brightness of beneficiated gray kaolin that was bleached with potassium permanganate and zinc hydrosulfite. For example, the data show that when potassium permanganate was used in amount of 5 pounds per ton of clay at room temperature and the zinc hydrosulfite was constant at 5 pounds per ton of clay, an additional 1.1% to 1.2% in GE. brightness was realized by pretreatment of the gray clay with titanium trichloride.

The data show also that all results were improved by carrying out the permanganate treatment at elevated temperature. However, when the gray clay was pretreated with titanium trichloride before the treatment with permanganate at the elevated temperature, an exceptionally bright clay (89.9% brightness) was prepared.

Data in Table I also show that 0.48 pound per. ton of titanium trichloride and 5 pounds per ton of potassium permanganate gave practically the same result obtained with pounds per ton potassium permanganate in the absence of titanium trichloride. In etfect, the use of 0.48 part byweight of titanium trichloride had a similar effect to the use of 5 parts by weight of potassium permanganate.

Data in Table I for the test in which the titanium trichloride was added after the potassium permanganate was agitated with the pulp for an hour show that the final clay product had the same brightness as the clay product obtained with similar processing but without addition of titanium trichloride. Thus, the titanium trichloride was not effective when it was added to the aqueous pulp after the potassium permanganate had reacted with impurities in the clay.

EXAMPLE II Another series of oxidation-reduction bleaching tests was performed on a different sample of flotation beneficiated gray Georgia clay. The processing of the clay was substantially as described in Example I. The brightness of the clay after flotation was 84.9%. The results, summarized in Table II, show that this gray kaolin could be brightened to an exceptionally high brightness value of 90.5% to 90.6% by the process of this invention.

Table II EFFECT OF TITANIUM TRICHLORIDE ON THE BLEACH- ING OF FLOTATION BENEFICIATED GRAY KAOLIN CLAY WITH POTASSIUM PERMANGANATE AND ZINC HYDROSULFITE 1. A method for bleaching an iron-stained oxidized mineral which comprises forming an aqueous pulp of said mineral, incorporating into said pulp a small amount of a water-soluble titanium compound, incorporating a water-soluble permanganate into said pulp, agitating said pulp until said permanganate is reduced, and then bleaching said pulp with a water-soluble hydrosulfur-ous bleaching reagent.

2. The method of claim 1 in which said mineral is a silicate mineral.

3. The method of claim 1 in which said mineral is clay.

4. The method of claim 1 in which said mineral is kaolin clay.

5. The method of claim 1 in which said mineral is gray kaolin clay.

6. The method of claim 1 in which said titanium compound has a valence less than +4.

7. The method of claim 6 in which said titanium compound is TiCl 8. The method of claim 7 in which said TiC1 is employed in amount within the range of 0.05 to 2.0 pounds per ton of mineral being bleached.

9. The method of claim 1 in which said titanium compound is TiCl employed in amount within the range of 0.05 to 2 pounds per ton of mineral being bleached, said permanganate compound is KMnO employed in amount within the range of 1 to 10 pounds per ton of mineral being bleached and said pulp is agitated with said permanganate at elevated temperature.

10. In a method for brightening gray kaolin clay, the steps comprising forming an aqueous pulp of discolored gray kaolin clay, incorporating therein a water-soluble titanium salt, thereafter adding a water-soluble permanganate salt into said pulp and agitating said pulp until the permanganate is reduced and bleaching the pulp with [a water-soluble hydrosulfurous bleaching reagent.

11. The method of claim 10 in which said gray kaolin has been brightened partially by froth flotation of colored impurities before said titanium compound is incorporated therewith.

12. The method of claim 10 in which said titanium compound is TiCl 13. The method of claim 10 in which said titanium compound is TiOl employed in amount within the nange of 0.05 to 2.0 pounds per ton of said kaolin clay.

14. The method of claim 10 in which said titanium compound is TiCl employed in amount within the range of 0.05 to 2 pounds per ton of said kaolin clay and said permanganate compound is KMnO employed in amount within the range of 1 to 10 pounds per ton of said kaolin clay.

15. A method for brightening discolored sedimentary gray kaolin clay which comprises:

subjecting said clay to froth flotation in the presence of flotation reagents selective to colored impurities in said clay, whereby a partially brightened clay is obtained in the flotation tailings and the impurities are concentrated in the froth,

forming an aqueous acidic pulp of said flotation tailmgs,

incorporating TiCl in amount within the range of 0.05 to 2 pounds per ton of clay, thereafter incorporating KMnO in amount within the range of 1 to 10 pounds per ton of clay, agitating the pulp until the pulp is brown,

land bleaching the pulp with a water-soluble hydrosulfurous bleaching compound.

16. A method for brightening discolored sedimentary gray Georgia kaolin clay which comprises:

subjecting said clay to froth flotation in the presence of flotation reagents selective to titaniferous impurity in said clay, whereby 8. partially brightened clay is obtained in the flotation tailings and said impurity is concentrated in the froth,

forming an aqueous acidic pulp of said flotation tailmgs,

incorporating TiCl in amount within the range of 0.05 to 2 pounds per ton of clay, thereafter incorporating KMnO in amount within the range of 1 to 10 pounds per ton of clay, agitating the pulp until the pulp is brown,

land while the pulp is acidic, bleaching it with a watersoluble hydrosulfurous bleaching compound.

17. The method of claim 16 wherein said hydrosulfurous compound is zinc hydnosulfite. I

18. The method of claim 16 wherein said pulp containing said KMnO v is aged for about 48 hours before being bleached with said hydrosulfurous bleaching compound.

References Cited by the Examiner UNITED STATES PATENTS HELEN M. MCCARTHY, Acting Primary TOBIAS E. 'LEVOW, Examiner.

J. POER, Assistant Examiner.

Examiner. 

1. A METHOD FOR BLEACHING AN IRON-STAINED OXIDIZED MINERAL WHICH COMPRISES FORMING AN AQUEOUS PULP OF SAID MINERAL, INCORPORATING INTO SAID PULP A SMALL AMOUNT OF A WATER-SOLUBLE TITANIUM COMPOUND, INTERCORPORATING A WATER-SOLUBLE PERMANGANATE INTO SAID PULP, AGITATING SAID PULP UNTIL SAID PERMANGANATE IS REDUCED, AND THEN BLEACHING SAID PULP WITH A WATER-SOLUBLE HYDROSULFUROUS BLEACHING REAGENT. 